Not Applicable
The present invention relates generally to the suspension of conduits, and more particularly to the hanging of production and casing strings of piping used for the production of hydrocarbons. More specifically, the present invention relates to hangers employed for suspending wellhead casings and production tubing when electrical power is supplied through the wellhead to downhole equipment.
After a well has been drilled, the well must be completed before hydrocarbon production can begin. The first step in completing a well is the installation of casing pipe in the well. Wells usually require two or more concentric strings of casing pipe. A casing string is a long section of connected pipe that is lowered into the wellbore and cemented. Hydrocarbon wells typically require four concentric casing strings: conductor casing, surface casing, intermediate casing, and production casing. The various casings extend into the wellbore to different depths to protect aquifers, to provide pressure integrity and to ensure isolation of production formations. After cementing the production casing, a final string of tubing is typically run down the well bore.
All of the surface equipment that supports the various pipe strings, seals off the well, and controls the paths and flow rates of reservoir fluids is referred to as the wellhead. All wellheads have at least one casing head and casing hanger. If multiple casings are installed, the wellhead will have a casing head and casing hanger associated with each concentric string of casing. If a tubing string is employed, the wellhead will also have a tubing head and tubing hanger. Each string of casing and the tubing string hang from its respective head. The heads are usually stacked upon one another with the tubing head stacked above the uppermost casing head. Hangers are used within the various heads to ensure that its respective string is correctly located. With some applications, a single hanger may be used to hang a plurality of pipe strings from a single head. For example, U.S. Pat. No. 5,794,693 discloses a dual tubing string hanger, which is herein incorporated by reference. Typically, hangers also incorporate sealing devices or systems to isolate the casing annulus from the upper wellhead components.
Many hydrocarbon wells are fitted with permanent sensors, such as pressure and temperature sensors, which require electrical power to transmit signals from the sensors to a remote point at the surface. Hydrocarbon wells may also employ subsurface equipment, such as pumps or heaters, which may also require electrical power. In order to supply power to these subsurface pieces of equipment, electric current from a source outside of the wellhead must be transferred through the wellhead to the electrically responsive device. Electrical power can be supplied downhole by several methods, including electrical umbilical cords, rigid tubular conductors, or more recently via coiled tubing. No matter which method of power supply is employed, in order to transfer the power through the wellhead, the power supply must be transferred through either the tubing hanger or the casing hanger.
The extreme environmental conditions inside the wellhead coupled with the rough nature of completion operations often cause damage to devices used to supply electrical power. Damaged equipment can potentially lead to electrical short-circuits that can present a hazard to persons working around the wellhead. Since the majority of the wellhead equipment is constructed of conductive materials, an electrical short inside of the wellhead can charge the outer surface of the wellhead. Unprotected persons may be exposed to electrical shock if contact is made with the wellhead""s outer surface.
The present invention addresses the potential electrical hazards associated with supplying power downhole. Generally, a hanger is provided for either casing or tubing strings, or a plurality of casing or tubing strings, that electrically insulates the internal suspension means of the hanger from the external sealing means of the hanger. Electrical power is typically transferred through a hanger by way of the internal suspension means. If electrical umbilicals are used, connecting devices may be located within the internal suspension means for connecting an upper and lower umbilical and transferring power through the hanger. If a rigid tubular conductor is used, the suspension means may be ported to allow the passage of electrical feedthroughs or, in the case of tubing hangers, the tubing itself may be charged with electrical power. By insulating the suspension means from the sealing means, which is in communication with the external surface of the wellhead, any potential electrical hazard is eliminated.